1. Field of the Invention
This invention relates to a novel and improved X-ray generator and in particular to an X-ray generator which contains more than one cathode heating filament which, when energized, emits electrons. More particularly, this invention relates to an X-ray generator, which contains several cathode heating filaments, at least some of which are electrically controllable independently from the others. Such an X-ray generator may be used in the medical field for diagnostic purposes.
2. Description of the Prior Art
A multi-focal X-ray generator, the X-ray tube of which contains several independently controllable heating cathodes, is known in the art, e.g. from the German Pat. No. 406,067. In one embodiment of the known generator, the cathode heating filaments may be supplied by separate heating current sources. In another embodiment, two cathode filaments are serially connected, and there is provided a switch via which either one or the other cathode filament may be supplied from a common heating current source. Most commonly a-c sources are used. When energized, a cathode filament emits electrons. Under the influence of a high-voltage, these electrons are accelerated and directed to an anode, where they give rise to the emission of X-rays. The geometry of the cathodes and the heating current intensity are such that on the anode there are created focal points or focal spots which have different sizes, with regard to each other.
In an X-ray tube for an X-ray generator of this type, two electrical supply lines are required for the first cathode filament, and an additional supply line is required for each additional heating filament. For instance, to control independently two focal spots, three supply lines are necessary. With increasing number of cathode filaments, this requires high-voltage installations, in particular high-voltage supply lines and cables, of increasingly difficult performance.
For this reason, for instance in the medical field diagnostic X-ray devices are known which have as a rule a maximum of two independently controllable focal points.
In U.S. Pat. No. 3,649,861, for instance, there is disclosed a double focus X-ray tube, the cathode of which provides alternatingly a large and a small focal spot upon an anode. The cathode head contains a pair of substantially parallel electron focusing recesses in each of which is mounted a filament. The recesses are focused such that when the entire length of each filament is energized, focal spots of equal length are imposed upon the anode in side-by-side relation so as to create a unitary rectangular focal spot having an overall width of about two millimeters. One of the filaments is electrically connected so as to permit energization of only one-half its length independently of the energization of the other filament, whereby a single focal spot one millimeter in width may be imposed upon the anode. This X-ray generator also requires three supply lines leading to two cathode filaments to independently control two different focal spots.
Hence, in conventional X-ray generators for diagnostic purposes, one is usually limited to the use of an X-ray generator having only two focal spots in order to be able to get along with a minimum of supply lines in the high-voltage installations, particularly in the high-voltage cables feeding the X-ray tube of the generator. It would, however, be desirable to provide an X-ray generator which, for example, requires only two supply lines for two cathode heating filaments. A third supply line could then be used to energize independently a third cathode filament and thus generate a third focal spot of different size.